Rare earth metal-based permanent magnets, such as R—Fe—B based permanent magnets, have high magnetic characteristics and thus are used in various fields today. However, rare earth metal-based permanent magnets contain a highly reactive rare earth element: R and thus are susceptible to oxidation corrosion in the air. Therefore, in the case where they are used without any surface treatment, corrosion proceeds from the surface due to the presence of a small amount of acid, alkali, moisture, or the like, whereby rusting occurs, causing deterioration or fluctuation in magnetic characteristics. Further, in the case where such a rusted magnet is incorporated into a device such as a magnetic circuit, the rust may be dispersed and contaminate peripheral parts. In light of the above points, methods for forming a copper plating film on the surface of a rare earth metal-based permanent magnet as a film having excellent corrosion resistance have been employed in the past.
Generally, methods for forming a copper plating film are roughly divided into an electrolytic copper plating treatment and a non-electrolytic copper plating treatment. In the case where a non-electrolytic copper plating treatment is used to form a copper plating film on the surface of a rare earth metal-based permanent magnet, the rare earth element or iron, which is a constituent element of the magnet, may eluted into the plating solution and reacts with a reducing agent contained in the plating solution, promoting the formation of a copper plating film on the surface of the rare earth element or iron eluted into the placing solution; in order to prevent such a problem, it is important to control the plating solution, but this is not always easy. In addition, a plating solution for a non-electrolytic copper plating treatment is generally expensive. Therefore, in the formation of a copper plating film on the surface of a rare earth metal-based permanent magnet, a simple and low-cost electrolytic copper plating treatment is usually employed.
Various methods for forming an electrolytic copper plating film on the surface of a rare earth metal-based permanent magnet have been proposed in the past. The research group of the present inventors has also proposed, for example, in Patent Document 1, a method for forming an electrolytic copper plating film on the surface of a rare earth metal-based permanent magnet using an alkaline plating solution containing Cu2+ ions for an electrolytic copper plating treatment. The plating solution has blended therein, as a chelating agent having a high chelate stability constant with Cu2+ ions, an organic phosphoric acid having two or more phosphorus atoms, such as 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), or a salt thereof and, as chelating agent having a high chelate stability constant with Fe ions, gluconic acid or a salt thereof. They have also proposed, in Patent Document 2, a method for forming an electrolytic copper plating film on the surface of a rare earth metal-based permanent magnet using an alkaline plating solution containing Cu2+ ions for an electrolytic copper plating treatment. The plating solution has blended therein a chelating agent having a predetermined chelate stability constant with Cu2+ ions (HEDP, a salt thereof, etc.) and a chelating agent having a predetermined chelate stability constant with Fe3+ ions (pyrophosphoric acid, a salt thereof, etc.) under a predetermined alkaline condition. In addition, it is also possible to form an electrolytic copper plating film on the surface of a rare earth metal-based permanent magnet using a commercially available plating solution for an electrolytic copper plating treatment. However, with the recent expansion of the application field of rare earth metal-based permanent magnets, there is a demand for an improved method for forming an electrolytic copper plating film on the surface of a rare earth metal-based permanent magnet, for example, a method for forming an electrolytic copper plating film having improved adhesion.